1. Field of the Invention
The present invention relates to a signal processing device employed in an information processing integrated circuit, and, more particularly to a signal processing device and a signal transmitting method employed in an integrated circuit device having superconducting wiring for digital signals.
2. Description of the Related Art
In recent years, circuit integration has improved, and the need to improve the transmission velocity of a signal has increased. A hindrance to satisfying said need is the length of wiring for transmitting the signal.
A signal delay time per gate is determined by a signal delay in the wiring.
Standard metal or semiconductor wiring more or less involves electric resistance and capacitance, and a signal delay time in the wiring is mainly determined by the capacitance and resistance of the wiring.
It is possible to reduce the signal delay time by reducing the resistance in the wiring. This is the reason why recently discovered high-temperature superconducting ceramic materials that demonstrate superconductivity at a relatively high temperature, such as the temperature of liquid nitrogen, are expected to function as wiring material for integrated circuits.
When a pulse signal is provided to an input end of the superconducting wiring having no electric resistance, an output end of the wiring provides, in addition to a signal corresponding to the input signal, a continuous by-product signal, because the non-resistance wiring does not absorb energy. How to suppress such an unwanted signal, i.e., noise pulses are an important issue.
The superconducting wiring may be used to connect signal processing circuit elements with one another to fabricate a required signal processing device such as an integrated circuit, a memory circuit, and an operational circuit. When a proper pulse signal is provided to an input end of the wiring, an output end of the wiring provides an output signal involving irregular pulses. The cause of such irregular pulses has not been sufficiently analyzed yet. If the superconducting wiring provides such irregular output pulses with voltage fluctuations whenever it receives an input pulse signal, the signal processing circuit with the superconducting wiring may frequently malfunction to reduce its reliability. It is important, therefore, to prevent such irregular output pulses occurring in the signal processing device with the superconducting wiring.
To solve this problem, the inventors prepared signal processing devices with superconducting wiring and tested them to observe changes occurring in output pulse signals with respect to input pulse signals. In the tests, an input terminal of the signal processing device was connected directly to its output terminal by the superconducting wiring, and various operational elements disposed between the input and output terminals were omitted.
FIG. 1 shows a model of superconducting wiring used in the tests. An output end of the wiring is open.
Generally, the superconducting wiring demonstrates complicated electric characteristics. In standard aluminum, copper, or semiconductor wiring, inductance thereof is ignorable, so that it is only necessary to consider the resistance and capacitance thereof. In the superconducting wiring, however, resistance may be ignorable but inductance and capacitance act on each other in a complicated manner.
In the standard wiring, noise attenuates gradually. In the superconducting wiring, noise does not attenuate because there is no resistance in the wiring. The noise in the superconducting wiring may increase due to the inductance and capacitance.
In analyzing the superconducting wiring, it is difficult to uniquely determine a distribution constant of the wiring. Accordingly, the length of the wiring is divided into a plurality of unit lengths, and the inductance and capacitance of each unit length are estimated to determine an overall distribution constant. This procedure is necessary to carry out a SPICE analysis method to be explained later.
The circuit of FIG. 1 is an equivalent of a concentrated constant circuit formed by dividing the superconducting wiring into 100 sections, i.e., by dividing the overall inductance and capacitance (distribution constants) of the wiring by 100.
FIG. 2 is a schematic view showing a wiring structure devised for analyzing the superconducting wiring. In the figure, superconducting wires X form a part of a 64-megabit memory. Each wire X has a width w of 0.3 .mu.m, a thickness t of 0.3 .mu.m, and a space s of 0.5 .mu.m. The thickness XOX of an SiO.sub.2 film between the wire and a grounding conductor is 0.2 .mu.m. The length a of the wire is 1 cm. According to plane parallel plate approximation, the capacitance C of the wire is 0.7 pF/cm and inductance L thereof 6.0 nH/cm.
FIG. 3 shows a waveform of an input pulse used for the analysis according to the invention. The waveform has a rise time tR of 10 ps, a fall time tF of 10 ps, a half-value width tPW of 500 ps, and a pulse height Vo of 5 V.
The pulse of FIG. 3 is applied to an input end of the superconducting wire of FIG. 2, and a pulse waveform appearing at an output end thereof is simulated and analyzed. The simulation is made according to the SPICE (Simulation Program with Integrated Circuit Emphasis), which is software developed and published by Stanford University of the U.S.
FIG. 4 shows transmission characteristics of the superconducting wire having an open output end, simulated according to the SPICE. In the figure, a continuous line represents the input waveform, and a dotted line an output waveform. When the input signal is applied to the input end of the wiring for a period of T1, the output end sequentially provides wide pulses A at regular intervals during the period T1. Even after the input signal is stopped, the output end continuously provides narrow pulses B during a period of T2.
Under these circumstances, the input signal will never be correctly transmitted through the superconducting wire. It is necessary to suppress the noise output pulses produced on the input signal.
To suppress such irregular noise pulses, a resistor RL is added to the output terminal to terminate the open end of the wiring.
FIG. 5 shows an equivalent circuit of the superconducting wiring whose output end is terminated with the resistor RL. The impedance of the resistor RL is the same as the characteristic impedance of the wiring. Namely, the output end of the superconducting wiring of FIG. 1 is terminated with a resistor element whose impedance is identical with the characteristic impedance of the wire. Resistance of the resistor RL is expressed as follows: EQU RL=SQRT(L/C)=92.6 .OMEGA.
FIG. 6 shows simulated transmission characteristics of the superconducting wiring of FIG. 5, terminated with the resistor having the same impedance as the characteristic impedance. In FIG. 6, noise pulses are suppressed, and an output pulse substantially correctly reflects an input pulse, because the output end is terminated with the resistor having the same impedance as the characteristic impedance of the wiring.
In this way, when the output terminal of superconducting wiring is terminated with the characteristic impedance, a signal is transmitted through the wiring with high fidelity. In addition, the zero electric resistance of the superconducting wiring is well utilized to minimize a delay time of the wiring. In the termination resistor, however, a current flowing through the resistor is consumed to produce heat and cause power loss. This heat may increase the temperature of the circuit over a transition point of the superconducting material, and destroy the superconducting state. The wiring of FIG. 5 consumes energy of 13.4 .times.10.sup.-12 J per pulse to produce heat.